Manipulation of nitrogen-contaminated natural gases



Nov. 8, 1960 1.. s. TWOMEY 2,959,022

MANIPULATION OF NITROGEN-CONTAMINATED NATURAL GASES Original Filed Aug.4, 1949 4 Sheets-Sheet 1 T 1 l A N i i JV-'7 i ,1 i 1 1 l i i m 3 V G i1 g I I T Q I 1 a I M l i LC! i A r-q--- I LL l w m L Ll.

MIMI! INVENTOR.

L S. TWOMEY 15 'P MW. 441/ AT RNEY Nov. 8, 1960 MANIPULATION OFNITROGEN-CONTAMINATED NATURAL GASES Original Filed Aug. 4, 1949 TWOMEY 4Sheets-Sheet 2 BY B01 T RNEY Nov. 8, 1960 L. s. TVWOMEY 2,959,022

MANIPULATION 0F NITROGEN-CONTAMINATED NATURAL GASES Original Filed Aug.4, 1949 4 Sheets-Sheet 3 NITROGEN VENT III] I; II I! In 1 Iihl oINVENTOR. L. S. TWOM EY A NEY L. S. 'TWOMEY Nov. 8, 1960 MANIPULA'I'IONOF NITROGEN-CONTAMINATED NATURAL GASES 4 Sheets-Sheet 4 Original FiledAug. 4, 1949 INVENTOR. L. 3. TWO M EY BY AT RNEY States M MANIPULATIONOF NITROGEN-CONTAMINATED NATURAL GASES This application is a division ofmy copending application Serial No. 108,631, filed August 4-, 1949, nowPatent No. 2,696,088, and having the same title. It relates to thetransportation, purification, storage and distribution of naturalhydrocarbon gases initially co'ntaminated by material proportions ofnitrogen.

Certain portions of the United States, notably western Kansas,southwestern Colorado, and the Texas Panhandle, produce great quantitiesof natural gas containing up to forty percent by volume of nitrogen. Thegreat part of this gas finds a market only at a considerable distancefrom the field and must be transported through pipe lines for hundredsof miles, at a cost which oftenmaterially exceeds the value of the gasat the well-head.

The separation and rejection of part or all of the original nitrogencontent has important advantages, even when this step is performed atthe delivery end of the transmission line, and evengreater advantageswhen the removal is efiected before the gas is transported over a greatdistance. The step is particularly effective and advantageouswhencombined with storage of part of the purified gas at a pointmore orless adjacent to that at which the gas is distributed and used, or whenthe step of purification is combined with the recovery of liquidhydrocarbons from the purified gas. The nature of these advantages andthe various manners in which they may best be realized will be referredto in detail hereinafter.

Various methods for separating the contaminating nitrogen from thenatural gas are available; the present specification describingonlythe'general method in' which separation is effected by liquefactionof the eritirefee'd stream and fractionation of the resultant liquid ina suitable column. This methodv of separation may be employed either inthe field or at the delivery" end of a long distance transmission line,or at some convenient intermediate point, and maybe combined 'witlistorage of part or all of the purified gas and with the recovery'ofvaluable liquid'hydrocarbonsfrom th'e'gasr The: invention may" best bedescribed wit-Itreference to the attached drawings and thefollo'wing'description thereof, in which! Fig. 1 is a diagram illustrating theess'ential steps of the process, devoid of d'e'ta'il' anddescribingvarious permissible. alternatives of-procedure;

Fig. 2 is aflow sheet of an 'operationand assemblage of apparatus forperforming the" actual separation of nitrogen and providing theextraneous refrigeration re quired by the system; the purified gas-beingstored in liquid form;

Fig. 3 illustrates a modification of the-operation in which the purifiedgas is-obtainedin 'the'fornr ofavapor at lowpressure-andis'recompressed'for'delivery into 'a long distancetransmission 1ine',a'nd" Fig. 4 illustrates'anoth er modification inwhich the purifiedgas, delivered from the column as a liquid,.is pumpedin liquid form through a'vaporizinginterchanger aren't 2 and thusdelivered into the transmission line under the pressure created by theliquid pump. H 7

Referring first to Fig. 1, A indicates a gas field producingnitrogen-contaminated natural gas; B is a dehydrating unit in which thegas is deprived of carbon dioxide, hydrogen 's'ulfid and water vapor; Cis a fractionating system as described detail in connection with Figs.2, 3 and 4; D is ges storage system and E is a distributing'systeni suchas a city ga s service. The locations of elements A and E are, ofcourse, fiXed by circumstances and not controllable. A I The other threeprincipal elements may be located as convenient: thus, the treating unitmust be between the field and the fractionating plant but maybeiadjacent to either if they are separated; the fracti-onating plant andthe storage plant (if provided) may each be adjacent to the field oradjacent to the distribution area or at a medial point, and finally, thefractionating plant and the storage plant may be closely adjacent or maybe separated by any convenient distance. V H g Fig. 1 shows a line F Fconnecting the field with the fractionating plant,- with treating unit Blocated anywhere between A and C; a gas line Gl-I connecting thefractionating plant with the storage plant; a liquid line 0 connectingthe fractionating plant with the storage plant, usefiil only if thesetwo elements are closely enough adjacent to permit. thetransfer of aliquefied gas in liquid form; a gas line GKJ connecting thefractionating plant directly with distribution and bypassing storage; agas line I--] connecting the storage with distribution; agas line F -A1connecting the field with the distributionv area and bypassing storageWith such lines, of lengths determined by the relative loca tions of theunits, it is possible to take care of any desired alternatives ofprocedure. H I I The line indicated at Z is for the purposeofin'troducing a diluent gas as later described. Raw gas from the fieldmay be used as a" diluent by introduction to line I through line A2. I,H ,7

The fractionating system',illustrated in detail in late-r figuresconsists first of a series or group of interchangers L in which the feedga is refrigerated, the cooled or liquefied gas passing to aconventional fractionating column M, for example, one provided withbubble plates, in which a desired port-ion of the nitrogen f .the' rawgas is removed as a top cut, which-effects partof the cooling of thefeed gas and is vented at N The column bottoms, a liquid richer inhydrocarbons than the feed gas, may pass through arelatively shortconnection O, in the liquid'conditi on, to a liquid phase storage vesselP frorn which the liquid is withdrawnas required through a vaporizer Qto be sent to distribution. Orthe bottom liquid may pass through aconduit R to refrigerating unit L in which it is vaporiied in effectingpart of the cooling of the feed gas, passing thence in gaseous formthrough GH to low pressure gaseous storage S or high pressure gaseousstorage T, or through G-K--.l to the'di'stributing system Oi" a's athirdalternative, the column'bottoms may be dir'ecte d'to a refractionat'ingcolumn'U fronifwhicha britt'tjin cut consisting of propane and heavier haiaanbdnsis with drawn at V while the top cut, consisting mainly rofmethane and ethane," passes to conduit" G and thusto gaseous storage msor T,':..0r" to a 'reliqi1f'efi'e} which places it inliquid storageP, or"directto" distributionsys tern E. In instances in' whiclither'raeaanatibai plant and the storage'system' are" separated bya distahce we;

great'for the transniissionasj aliquid of'th e bot from column M,reliquefier w will take carelnot onl y of the topcut from colnmnU butalso (lithe-"Teapot:

ized bottom cut fror'n column M.

Any make-up refrigeration required in L will be supplied by theevaporation of a liquefied gas, such as liquid methane, introduced froman extraneous source at X and returned from Y to its source.

Referring now to Fig. 2, illustrating a method which delivers thepurified gas in liquid form into liquid storage: an actual pipe linesupply of nitrogen-contaminated fuel gas in taken by Way of example,this gas containing 90.7% of hydrocarbons, mainly methane, and 9.3% oflower boiling components, almost entirely nitrogen. In the ensuingdescription, percentages are in mol percents, pressures are inatmospheres absolute and temperatures in degrees Kelvin. All the figuresgiven are close approximations, fractions being substituted by thenearest round figure. It will be understood that the pressure andtemperature relations recited are illustrative and not limiting. Theywill vary to some extent with changes in compositionof the gas and maybe varied, within limits, at the will of the operator.

Gas from the field, previously substantially freed from carbon dioxide,hydrogen sulfid and water vapor, and at an assumed pressure of 12atmospheres, enters the system through the line 10, which willordinarily though not necessarily be the delivery end of a long distancetransmission line. This gas enters an interchanger 14 in which it iscooled to about 285 by separated nitrogen leaving the system. It thenpasses through conduit 15 to interchanger 16 in which its temperature isreduced to about 134 by an expanded and evaporating stream of liquidmethane produced by a cascade liquefying system later described. At thistemperature and at substantially the original pressure of 12 atmospheresthe gas is about 93% liquefied.

The partially liquefied stream passes through conduit 17 to a boilingand condensing coil 18 immersed in a pool 19 of liquid, substantiallypure methane in the base of a fractionating column 20 (column D of Fig.1). In this coil liquefaction is completed, the liquid stream passingthrough conduit 21 and expansion Valve 22 and entering the column at amedial height as at 23.

It should be understood that While liquefaction of the feed stream priorto entry into the column is desirable, as restricting the column to aminimum size, it is entirely possible to feed to the column a partiallyliquefied feed stream, or even a gaseous stream, in such cases theliquefaction requisite for fractionation being produced within thecolumn by increasing the quantity of reflux liquid.

The column may be of the single stage type and may be maintained at 3atmospheres pressure. With a sufficient number of effective plates, thetemperature in pool 19 will be about 125 and the vapor temperature atthe upper end of the column about 89 K. The composition of the vaporvented at 24 will be about 99% nitrogen and 1% methane.

The vent vapor is divided at the column outlet, a portion passingthrough conduit 25 to a nitrogen liquefying cycle later described Whilea quantity equal to that momentarily separated from the gas feed passesthrough conduit 26 to an expansion valve 27 by which its pressure isreduced to about 1 atm. and its temperature to about 86. The vent thenpasses through interchanger 14, in which its temperature is raised to290 in efiecting the first cooling of the gas feed, and is dischargedfrom the system at 28.

The column is provided with reflux liquid by a nitrogen liquefactioncycle taking gas from the top of the column through conduit 25. The gaspasses first through an interchanger 29 in which its temperature israised to 305 in cooling a compressed and water-cooled nitrogen stream,then through conduit 30 to two stages of compression 31 and 33 withinterposed water-cooling at 32 and final water-cooling at 34. Thewater-cooled stream, at a pressure of 25 atmospheres, is cooled to 129in interchanger 29 in heating the stream of nitrogen passing to thefirst stage of compression.

The refrigerated nitrogen then passes through conduit 36 to interchanger37 in which it is cooled to and is liquefied by interchange against coldmethane vapor from a source later described. The liquefied stream passesthrough conduit 38 and expansion valve 39, by which it is reduced tocolumn pressure, and enters the upper end of the column in which itfunctions as reflux liquid.

Returning now to the bottom of the column the liquid methane collectingin pool 19 passes through conduit 40 to interchanger 41 in which it iscooled by interchange against expanded and evaporating liquid nitrogendrawn from the nitrogen liquefaction cycle previously described. Thecooled liquid then passes through conduit 42 and expansion valve 43 toan insulated storage tank 44 which may be maintained at 1.15atmospheres, at which pressure the temperature of the liquid will beabout 113 K. The composition of the liquid entering the tank isapproximately 0.2% nitrogen and 99.8% methane and heavier hydrocarbons.In this tank the liquid is maintained in storage until required, atwhich time, it is Withdrawn through conduit 45 to be vaporized anddistributed. J

If preferred, the liquid may be withdrawn from storage by a pump 146adapted to handling liquids, by which it is raised to some requiredtransmission line pressure, then vaporized as at 147 and introduced intoa transmission line 148 leading to a distribution system.

Due to the reduction in pressure at expansion valve 43 there is a smallamount of flash from the liquid as it enters the vessel, usually about6% of its Weight. This vapor passes through conduit 46 and interchanger47, in which its temperature is raised to 305, then through conduit 48to a compressor 49 which raises the pressure to 3 atmospheres, through awater-cooling step 50, through interchanger 47 in which it is cooled toabout 132, and finally through conduit 51 to the column feed at 23.

The production of flash vapor in the storage tank may be avoided bysufficiently extending the aftercooling of the column liquid by expandedand evaporating liquid nitrogen, in which case elements 46, 47, 48, 49,5t) and 51 will not be required. This liquid may be drawn from conduit38 through branch conduit and expanded by valve 151 into interchanger41, the vaporized nitrogen returning through conduit 97 to a junctionwith conduit 25 leading to interchanger 29.

The refrigeration required in the above steps is provided in part by theexpansion of the gas feed from intake pressure to column pressure, inpart by the nitrogen cycle above described, and in part by a cascadesystem including an ammonia cycle, an ethylene cycle and a methanecycle.

Starting at the right hand end of Fig. 2, the ammonia cycle comprises atwo-stage compression unit 52 and 54 with intercooling at 53 andaftercooling at 55, the pressure being raised to about 4 atmospheres inthe first stage and to about 15 atmospheres in the second. At the latterpressure the ammonia is liquefied in the aftercooler at 311 and passesthrough conduit 56 into a receiver 57. The liquid then passes throughconduit 58 and expansion valve 59 into a flash tank 60 maintained atabout 4 atmospheres and 272 K. The flash from this tank returns throughconduit 61 to the intake of second stage compressor 54.

The flashed liquid ammonia passes through conduit 62 and expansion valve63, by which its pressure is reduced to 1.15 atmospheres and itstemperature to 245, to an interchanger 64 in which it liquefies ethylenein the next stage of the cascade. The ammonia vapor returns at about 260through conduit 65 to the intake of first stage compressor 52.

The ethylene cycle includes a two-stage compression init 6. 116 68. wi hinterse ting a 6 and at e o iu at 69, the pressure being raised to aboutatmospheres in the first stage and to 22 atmospheres in the second. Thecompressed gas leaves the aftercooler at 311 and passes through conduit70 to interchanger 64 in which it is liquefied at 248 by expanded andevaporating liquid ammonia. The liquefied ethylene passes throughconduit 71 into a receiver 72 and thence through conduit 73 andexpansion valve 74 into a flash tank 75 maintained at about 5atmospheres, and 201. The flash from this tank returns through conduit76 to the intake of second stage compressor 68.

The liquefied ethylene passes through conduit 77 and expansion valve 78,by which its pressure is reduced to 1.15 atmospheres and-its temperatureto 171, to interchanger 79 in which it liquefies methane in the thirdstage of the cascade, the ethylene vapor returning at about 260 throughconduit 80 to the intake of first stage compressor 66.

The methane cycle includes a two-stage compression unit 81 and 83 withintercooling at 82 and aftercooling at 84, the pressure being raised to6 atmospheres in the first stage and to 28 atmospheres in the second.The compressed gas leaves the aftercooler at 311 and passes throughconduit 85 to interchanger 86 in which it is cooled to 290 byinterchange with a returning stream of once-used methane. Thepartiallycooled gas passes through conduit 8'7. to interchanger 79, inwhich it is liquefied at 176 by an expanded andevaporating stream ofliquid ethylene.

The liquefied; met'hanepasses through conduit 88 to a receiver 89- andthence through conduit 90" and expansion' valve 91' to a flash tank 92maintained at 6 atmospheres and 139. The flash from this tank returnsthrough conduit 93 to the intake of second stage compressor 83;

The liquid methane thus produced supplies refrigeration to the raw gasliquefying and fractionating system at two points. i

A stream of the liquid passing from flash tank 92 through conduit 94 isdivided, the smaller portion passing through conduit 95 and expansionvalve 96, by which its pressureis reduced to 1.5 atmospheres. andjts.temperature to 118, to interchanger 37in whichitefiects' thedescribed-liquefaction of nitrogen, passing, thence through conduit 98to interchanger 86, in which it effects the first cooling of cascademethane vapor, and returningthrough conduit 99 to the intake of methanecompressor 81 at 270.

The remaining quantity of liquid methanepasses through conduits 94 and100 and expansion valve 101, by which its pressure is reduced to 1.4atmospheresand its. temperature to 117 K., tointerchanger 16 ,in which ipro uce the. esc bed parti l. ique ction q ithe dehydrated as ee The apr from this cxnansi na d interchange returns through conduit 102' to thefirst stage me h ne mp essor. 8

F o-which er nce s n wmade. illu t ate a modification of the method ofFig. 2 in which the purified gas is delivered into a transmission lineor distributing y em at co mn p ssur r a a ghe P ess ep oducedbyrecompression of the pro-duct gas.

The dehydrated gas supply enters the systemdhrough conduit 10,, and isdivided. into two streams passing in parallel, through interchangers.110 and 143. These Streams are. vcooled-to..about134" K. and partiallyliuefied by an expanded and evaporating stream of. the liquid, principallymethane, withdrawn from the bottom of fractionating. column 20 and by,vent nitrogen from the top of the column. The partially liquefiedstreamsare .mergedin conduit. 1 11, and. pass .to hqiling-and-condensing coil,19' in. which. liquefaction is completed except,for.possible..diflicultlyliqueiiable gases such as neon or helium, thelatter a fairly common componentime nitrogen-containing natural gases.If these are present the; tte mrnay he passed through conduit 112 to aseparator 118 from which uncondensed gases are vented at 114, The liquidthen passes through conduit 21 and expansion valve 22, bywhich it isreduced to column pressure, to the medial point 23 at which it isintroduced into the column. As before described, this may be a single.stage column, provided with bubble plates, or other form offractionating column as may be preferred, and is desirably maintained atabout 3 atmospheres absolute.

The liquid collecting in the bottom of the column, consisting of thehydrocarbons originally present in the gas together with aminute residueof nitrogen, passes in greater part through. conduit 115 and expansionvalve 116, by which it is reduced to slightly over atmospheric pressure,to interchanger 110, in which it is heated to approximately thetemperature at which the feed gas enters the system, The warmed gaspasses through conduit 117 to a gas compressing unit generally indicatedat 118, in which the pressure is raised to that required to introducethe gas into a long distance transmission line 119. In case thefractionating plant is located at the delivery end of the line a singlestage compression unit at 118 may suffice for introducing the gasdirectly into a distribution system or into gaseous storage for laterdistribution.

The remainder of'the liquid. stream from the column is diverted throughbranch conduit 120 and expansion valve 121: into interchanger 122 inwhich it is vaporized and brought 1113110 substantially atmospherictemperature in liquefying a stream of compressed nitrogen. The warmed.gas from this interchange passes through conduit 123. to a point. ofjunctionwith, conduit 117 and thus to compressor 118.

Liquid nitrogen forv reflexingthe column is provided by anitrogenliquefaction cycle difiering somewhat from that described inconnectionwith Fig. 2. The. stream of cold. nitrogen leaving the. topof. the column at 24is divided, a-. quantity suflicient to provide thereflux required by the column passingthrough conduit 25 to interchanger29, in whichit is brought up to atmospheric temperature, and thencethrough conduit 30 to the'twostage compression: and cooling unit31,e32.33+34 in which, the. pressureis raised to. 25 atmospheres. Thecompressed. stream. flow-ingthrough conduit 35is divided, one portion.passing, through valve 124 into. interchanger 122 in.which it iscooledand liquefied by anexpanded stream of column-bottom. product. Theremaining portion. passes. through valve. 125 intointerchanger' 29; inwhich it.is cooledby the nitrogen stream passing toward the compressor,the, cooledstream passing. through conduit 36 tointerchanger 37 in whichhis liquefiedby an expanded. and evaporating. stream of liquid methane.The two streams of liquid nitrogen pass through conduits 126;.and 38. toexpansion valve 39'andthus into the upper end of the column.

Dependent orrthe, composition of the feed gas and-the closeness of.fractionation, three alternatives are available in the handling of thenitrogen stream-entering conduit-127. In the first alternative, theentire stream may pass through conduit 141, expansion valve 142 andexchanger 143- and be discharged through nitrogen vent 144. In exchanger143 it is heated by interchange with a portion of the gas feed, which istherebycooled-and joins, in conduit 111, the portionof the gasfeedcooled in exchanger 110.

In this alternative, the vent nitrogen hasnopart 'in liquefyingthecascade ethylene, which is-liquefieclby exchange with boiling liquidammonia in exchanger 64, the parallel interchanger 129 (used in thesecond alternative) being then inoperative.

As it is desirable to control rather closely the enthalpy of the mergedstreamentering coil 18 from conduit 111, there are some conditions offeed composition and closeness of fractionation under which it isuneconomical or impossible to pass all or even any part of the ventnitrogen from conduit 127 through exchanger 143. This leads to thesecond alternative in which a portion of the vent nitrogen takes thepath 'just described, while the remainder passes through expansion valve130 into interchanger 129, where it is heated in liquefying either aportion or all of the cascade ethylene, thence passing through conduit131 to nitrogen 132. In the event that the quantity of nitrogenavailable for passage through 129 is insuflicient to liquefy all of thecascade ethylene, the excess of ethylene is liquefied by exchange withboiling liquid ammonia in exchanger 64.

In the third alternative, the entire quantity of vent nitrogen is passedthrough exchanger 129 and is heated in liquefying ethylene, finallypassing out through nitrogen vent 132, exchanger 143 meanwhile beinginoperative, with all of the gas feed passing through interchanger 110.In this alternative, ammonia may or may not be required in exchanger 64for liquefying part of the ethylene, depending on the amount of ethylenewhich the vent nitrogen is able to liquefy in exchanger 129.

The cascade system of Fig. 3 differs from that of Fig. 2 in both themethane and the ethylene liquefying stages. In Fig. 2 a single ethyleneinterchanger 64 is provided, the liquefaction of compressed ethylenebeing produced solely by expanded and evaporating liquid ammonia. InFig. 3 the stream of compressed ethylene delivered by compressor 68 isdivided, a portion passing through valved conduit 71 to interchanger 64,in which it is liquefied by boiling ammonia. The remainder of thecompressed ethylene passes through a valved branch conduit 128 into aninterchanger 129 in which it is liquefied by gaseous nitrogen flowingfrom column 20 through conduit 127 and expansion valve 130. Thenitrogen, warmed by this interchange, passes out of the system asdescribed. The ethylene liquefied by these interchanges flows throughconduits 71 and 133 into receiver 72, thereafter taking the coursepreviously described.

The methane cycle diifers from that of Fig. 2 in two respects. Thus,interchangers 86 and 79 are arranged in parallel, both delivering liquidmethane into receiver 89, interchanger 86 being supplied with gaseousmethane through branch conduit 133 and draining through conduit 134.Nitrogen liquefier 37 is cooled by liquid methane passing to it fromflash tank 92 through conduit 135 and expansion valve 136, the methanevapor resulting from the interchange returning to compressor 81 throughconduit 98, interchanger 86 and conduit 99. The cooling of the nitrogenliquefier is the only use of liquid methane in this modification of theinvention and the capacity of the cascade system is correspondinglyreduced.

Fig. 4 illustrates a modification of the invention in which a pumpadapted to raise liquefied gases to a high pressure replaces thecompression unit 118 of Fig. 3, the column liquid being passed through avaporizing interchanger on its way to the intake of a transmission line.

Referring to Fig. 4, the liquefaction of the gas feed is efiected ininterchangers 14 and 16 and condensing coil 18 by interchanges withgaseous column nitrogen, expanded cascade methane and boiling columnliquid, the first two interchanges being in parallel and the third inseries with the two. Thus, the feed stream passing through conduit isdivided between interchangers 14 and 16 in proportion to the amount ofrefrigeration available in each, the first being cooled by vent nitrogenfrom the column, passing through conduit 26 and expansion valve 27 andbeing vented, after warming by interchange, through nitrogen vent 129.The second interchanger 16 is cooled by cascade methane passing throughconduits 94 and 100 and expansion valve 101, the expanded and warmedmethane returning to compressor 81 through conduit 102. The refrigeratedgas, which may be partly liquid, leaves the interohangers throughconduits and 17, the latter leading to coil 18 from which it passesthrough conduit 21 and expansion valve 22 into the col- 8 umn. The feedentering the column may be wholly liquid, or partially liquefied, oreven gaseous, liquefaction in the column of any vaporous feed beingproduced by increasing the supply of reflux liquid over that requiredfor flactionating a liquid feed. I

The liquid collecting in the column, consisting of normally gaseoushydrocarbons together with a reduced and ordinarily very smallproportion of nitrogen, is preferably cooled below column temperature ininterchanger 41, passing thence to a pump 140 capable of raising it, inthe liquid form, to whatever pressure is required at the transmissionline intake. The liquid passes from the pump through conduit 46 tointerchanger 86, in which it is vaporized without substantial change inpressure in liquefying part of the required supply of cascade methane,the high-pressure purified gas passing thence directly into transmissionline 119.

The initial cooling of the reflux nitrogen is effected by gaseous columnnitrogen on its way to compressor 31, as previously described, and theliquefaction of the cooled nitrogen by expanded cascade methane drawnfrom conduit through branch conduit 137 and expansion valve 138 andreturned to compressor 81 through conduits 139 and 99.

The cascade system of Fig. 4 is identical with that of Fig. 2 in theammonia and ethylene cycles and with that of Fig. 3 in the methaneliquefying cycle.

Numerous and important advantages are realized from the removal of amaterial part of the nitrogen prior to transmission of the gas to adistant point:

(a) The therm transmitting capacity of any given pipe line is increasedby the removal of the inert diluent and the concentration of theoriginal fuel value of the gas into a smaller volume and weight;

(b) The horsepower required to transmit the gas over a long distance isconsiderably reduced, both by reduction in quantity of gas which must betransmitted per unit of heat transmitted and by reason of the more readycompressibility of the hydrocarbon-enriched residue;

(c) An important saving in cooling water consumption is effected, byreason of the higher temperature of nitrogen at any given dischargepressure and the elimination of the nitrogen;

(d) The thermal storage capacity of the line itself and of anyadditional storage vessels which may be provided are increased inproportion to the quantity of nitrogen removed;

(2) The removal of the nitrogen, if present in material proportion inthe field gas, often or usually permits the separation and recovery as asalable product of the higher boiling hydrocarbons (propane and heavier)without reduction in the heating value of the gas or with themaintenance of a specified B.t.u. requirement;

(f) The removal of higher boiling hydrocarbons thus permitted oftengreatly improves line operating conditions, avoiding risk ofcondensation in and trapping of the line;

(g) The removal of nitrogen permits the use of gas from fields of whichthe product is initially of too low heating value to be usefulcommercially;

(h) The removal of nitrogen permits standardization of heating value ofa gas supply drawn simultaneously and in varying proportions from fieldsor wells producing gases of different compositions;

(i) The removal of nitrogen and consequent increase in calorific valuepermits attainment of higher flame temperatures which, in manyindustrial operations greatly increases the efliciency ofhigh-temperature heating steps, by increasing the range between theflame temperature and the temperature to which the work must be brought;

(j) The removal of nitrogen makes it possible to increase the averagetherm load factor of the transmission line by permitting it to care fora largeraverage distribution load. 1

The step of. removing-nitrogen fromcontarninated natural gas is,particularly desirable. in instancesin which any. part ofthe gassupplyis, to, :be stored in liquid form, thus:

(k) The storage of inert material is avo-ideda given vessel willhs srnsr th me by Pa e a the concentration of th original heait g yalu einto a'smaller liquid volume; i l v (I) In the more. usual instance,inwhich the. gasisdeprived of nitrogen before, long distance transmissionand "reliquefied for. storageatthe. delivery. end of the line, thequantity of heatto beremoyed/ in the reliquefying plant is reduced;

(m) T he temperatures. ofreliquefaction and of storage are materiallyraised, avoidingthe-u se. of e extremely low temperatrues. which are,the. most costly to at tain;

(n) Elevation of the. temperature of the liquid-storage vessel reducesheat infiltration through. any given vessel insulation. and

(o) Reduces embrittlementof ferrous materialsused in storage vessels; ii l (p) Change in composition of the liquid, which follows fromfractional vaporization of nitrogen, is eliminated by nitrogen removaland (q) Materially less heat is required for the vaporization of theliquid when required for use in the gaseous phase;

(r) The minimum temperature encountered in revaporizing the liquid isincreased and the liability to freezing of the heating fluids used invaporizing is reduced.

The steps of removing a material proportion of nitrogen prior to longdistance transmission and of placing part of the transmitted gas instorage at the delivery end of the line, at times of less than averagedemand, to be drawn on to help meet demands greater than average, arehighly cooperative. Not only does the re moval of the nitrogen increasethe transmitting capacity of the line and the storage capacity of boththe line and the delivery-end storage, thus permitting smaller pipelines and storage units to carry a given load, but also the provision ofstorage capacity materially improves the functioning of thenitrogen-removal plant.

Demands of a distribution system for gas vary widely from day to day oreven from hour to hour, this variation being seldom less than three toone and often much greater. This variation in demand has, in the past,been compensated in various ways, as for example by packing the line(raising the line pressure and thus increasing the quantity of gas intransit), and cutting off socalled interruptible loads at times ofincreased domestic demand (involving major price concessions to suchindustrial users) and similar expedients.

Both the transmission line and the nitrogen removal plant function mosteconomically under an unvarying load. The provision of storage at thedelivery end of the line, in quantity sufficient to supply thedifference between average demand and maximum demand for the anticipatedperiod, permtis the pipe line to deliver, and therefore to take from thenitrogen removal plant, a constant quantity. With this provision, boththe nitrogen removal plant and the pipe line need be only of suchcapacity and dimensions as to carry the average load, rather than themaximum, and both first cost and operating cost are reduced.

Or, for a transmission line of fixed size, equipped with a storagefacility, the average therm load factor of the line can be increasedthrough its increased ability to meet peak demand, which enables it tosupply an increased average demand, because the average demand which theoperator may commit the line to supply is limited by the ability of thesystem to meet peak demands successfully.

In removing a relatively large proportion of nitro- "1'0 gen, even withsimultaneous. separation .ofv valuable liq,- uids of'high heating value,it may. occur that the calorifig; valueof the residual. gasis raisedabove. that required by local custom. or ordinauges. In such cases the.overly rich gas. may bediluted back. to. therequirement by. theadmixture of gases 'oflower or no heating value, for example, coke oyengas, producer gas, nitrogencontaminated: natural gas, combustion gasesor air.

Given a source of fuel gas of relatively high thermal value and, at adistance therefrom, a fluctuating demand for a gas of relatively lowthermal value, economy in the investment and operating cost can beattained by transmitting the high thermal value gas to a point as closeas -is convenient to the demand, storing, in times of reduced demand, aportion of the transmitted gas in its undiluted state, meanwhilediluting another portion of the transmitted gas with air flue gas, coalgas, coke e as; p duce sea Wat a O n ab e terial in v order to reduceits thermal value to that required by the demand, and supplying thedemand with the dilut- 9 a ,1 tim f nc e ed d m n hi h. h rm value gascan be removed from storage, diluted as above, and directed to thedemand to augment the supply available by diluting the current deliveryof the line. In event of line delivery interruption, the gas removedfrom storage and diluted can entirely replace line delivery anddilution.

With a line of fixed size, the addition of such a storage facility andoperation according to the above method will permit the increase of theaverage therm load factor of the line, as compared with the transmissionof diluted gas with or without storage, or with the transmission of richgas, its dilution and storage in the diluted condition of a portion forsubsequent use.

Provisions are made for this operation in the showing of Fig. 1 by thecross-over line Z connecting the field line F with conduit GH leading tothe storage plant and conduit G--HK leading to the distribution system,and by the injecting connection 2' into cond-uit J ahead ofdistribution.

I claim as my invention:

1. A manipulation of natural hydrocarbon gas initially contaminated withnitrogen, comprising: refrigerating said gas; subjecting therefrigerated material to a fractionation employing a reflux liquid andthereby separating a vapor enriched in nitrogen from a liquid enrichedin hydrocarbons; passing a portion of said vapor in heat interchangewith a compressed gas and thereby liquefying said compressed gas toproduce a liquid refrigerant; evaporating said liquid refrigerant inheat interchange with a second compressed gas to produce a second liquidrefrigerant having a lower boiling point than first said liquidrefrigerant, and evaporating said second liquid refrigerant in heatinterchange with another portion of said vapor to produce at least aportion of the reflux liquid for said fractionation.

2. In the fractionation of a refrigerated natural gas initiallycontaminated with nitrogen, in which a vapor enriched in nitrogen isseparated from a liquid enriched in hydrocarbons, the steps comprising:providing reflux liquid for said fractionation by liquefying a portionof said vapor by heat interchange with boiling liquid methane; producingsaid liquid methane by heat interchange between a stream of compressedgaseous methane and a stream of boiling liquid ethylene, and producingat least a portion of said liquid ethylene by heat interchange between astream of compressed gaseous ethylene and another portion of the vaporproduced by said fractionation.

3. In the production of a stream of liquid refrigerant predominantlymethane, the steps comprising: compressing a stream of warm gasconsisting principally of methane; dividing the compressed stream intobranch streams; liquefying one of said branch streams by heatinterchange with a boiling liquid refrigerant; liquefying a second "11branch stream by heat interchange with a stream of expanded refrigerant,predominantly methane, from a previous use; merging said branch streamsto form a stream of liquid refrigerant, and expanding and heating aportion of last said stream in the previous use aforesaid.

4. In the production of refrigeration at low temperatures the stepscomprising: compressing a stream of methane gas in a plurality of stagesand cooling the stream to remove the heat of compression; refrigeratingand thereby liquefying said compressed stream; producing a first vaporstream and a residual liquid by expanding said liquid stream to thepressure of an intermediate compression stage; separating the residualliquid and then producing a refrigerating effect by vaporizing theresidual liquid to produce a second vapor stream; interchanging at leastone of said vapor streams with the cooled compressed stream to supply atleast part of the refrigeration required for liquefaction thereof;conducting the first vapor stream to an intermediate compression stageand conducting the second vapor stream to the initial compression stage.

5. The process of claim 4 in which said refrigeration effect is utilizedat least in part to liquefy a stream of gas predominantly nitrogen, andconducting the resultant liquid :to a fractionating operation to serveas reflux therein.

, References Cited in the file of this patent UNITED STATES PATENTS1,843,043 Patart Jan. 26, 1932 2,082,189 Twomey June 1, 1937 2,180,435Schlitt Nov. 21, 1939 2,265,558 Ward et a1. Dec. 9, 1941 2,424,201 VanNuys July 15, 1947 2,475,957 Gilmore July 12, 1949 2,500,118 Cooper Mar.7, 1950 2,508,821 Gammill May 23, 1950 2,534,478 Roberts Dec. 19, 19502,541,569 Born Feb. 13, 1951 2,557,171 Bodle June 19, 1951 2,600,110Hachmuth June 10, 1952 2,658,360 Miller Nov. 10, 1953 2,677,945 MillerMay 11, 1954

